1. Field of the Invention
The present invention relates to a receiving circuit of a mobile communication terminal, and more particularly, to a circuit for preventing the deterioration of receiving sensitivity caused by an intermodulation (IMD) product signal.
2. Description of the Related Art
An intermodulation product signal generated from a nonlinear active element often appears within the frequency band of a Code Division Multiple Access (CDMA) receive signal. This IMD product signal lowers the receiving sensitivity of a mobile communication terminal such as a CDMA or Personal Communication Service (PCS) terminal.
FIG. 1 is a block diagram of a conventional receiving circuit of a mobile communication terminal using a CDMA system. The receiving circuit includes a front-end receiver 116, an intermediate frequency (IF) band pass filter (BPF) 110, an automatic gain controller (AGC) 112, and a demodulator 114. The front-end receiver 116 has a low-noise amplifier (LNA) 100, a radio frequency (RF) band pass filter (BPF) 102, a mixer 104 for converting an input RF signal into an IF signal, a local oscillator 106 for supplying an appropriate frequency to mixer 104, and an IF amplifier 108 for amplifying the IF signal generated from mixer 104. The IF BPF 110 passes only a specific band of the IF signal generated from front-end receiver 116. AGC 112 automatically controls the gain of the IF signal passing through BPF 110. Demodulator 114 demodulates the IF signal generated from AGC 112.
FIG. 2 illustrates a frequency spectrum of an intermodulation product signal generated during reception of a CDMA receive signal through front-end receiver 116 shown in FIG. 1. A multi-tone interference signal is received together with the CDMA receive signal. This interference signal is intermodulated while passing through the front end receiver 116, thereby producing intermodulation product signals (IMD products) that fall within the band of the CDMA receive signal. These IMD products, which are typically third order and fifth order products, act as noise in the CDMA receive signal.
Receiving sensitivity of a CDMA mobile communication terminal can be improved by employing a technique for reducing the intermodulation product signal serving as noise in the CDMA receive signal. This technique is very important to the performance of a CDMA mobile communication terminal. As one example of such a technique, a variable attenuator is employed. The attenuator is installed at the front end receiver 116 of the receiving circuit and attenuates an interference signal by a predetermined amount controlled by a controller. As a second example, a low noise amplifier having a variable gain is used. If an interference signal is received, an intermodulation product signal can be prevented from being generated at the next stage by reducing the gain of the low noise amplifier, so that it operates in a more linear region. In a third technique, the power of the local oscillating signal driving the mixer is varied. As the power of the local oscillating signal driving the mixer increases, an intermodulation product signal generated from the mixer decreases in amplitude. In yet another technique, the driving current of the low noise amplifier is controlled. As the driving current for operating the low noise amplifier increases, the amplitude of the intermodulation product signal decreases.
One problem with the variable attenuator approach is that the variable attenuator attenuates both the interference signal and the desirable CDMA information signal. Therefore, the receiving sensitivity of the receiving circuit of the mobile communication terminal is lowered. If the low noise amplifier with variable gain is used, the input/output impedance of the amplifier varies as the gain decreases, thereby deteriorating circuit performance. Moreover, there is the possibility of the circuit oscillating as a result of such gain variation. In the third example of controlling the power of the local oscillator, the attenuated range of the intermodulation product signal is narrow. In particular, the intermodulation product signal generated from the low noise amplifier positioned before the mixer can not be attenuated with this method. In the fourth example of controlling the driving current for operating the low noise amplifier, the attenuated range of the intermodulation product signal is narrow and there is a strong possibility that the circuit within the next stage will oscillate.
It is therefore an object of the present invention to provide a circuit for reducing or effectively eliminating an intermodulation product signal generated from a front-end receiver of a receiving circuit.
In an illustrative embodiment of the invention, there is provided a receiving circuit of a mobile communication terminal including a front-end receiver for receiving a CDMA signal from an antenna and converting the CDMA signal into an intermediate frequency signal. A feed forward linearizer includes an intermodulation product generator for generating essentially only an intermodulation product signal of an interference signal contained in the CDMA signal, a phase shifter for phase shifting the intermodulation product signal generated by the intermodulation product generator, and a variable attenuator for adjusting the amplitude of the intermodulation product signal generated by the phase shifter. A controller controls a phase shifter so that a phase difference between the intermodulation product signal generated from the intermodulation product generator and an intermodulation product signal generated from the front-end receiver may be 180xc2x0. The controller also controls the variable attenuator so that these intermodulation product signals may have the same magnitude. The IMD products are then essentially canceled via a combiner.